There is plenty of solutions out there to do chat messaging "kinda like" or maybe better than Slack. I'm using Slack daily and I can't find what this platform has something so magical about it.
I am personally building a communication client that can do IM, video-conferencing, file sharing, posting articles… all based on XMPP. It works in the browsers and we are proposing now apps for Android and macOS/Windows/Linux. It's way lighter than Slack, can be deployed easily on a VPS or in your company and is fully compatible with many other XMPP clients/bots (Github is proposing a XMPP bot for MUCs for example :)). If you want to try it out or know more: https://movim.eu.
If you don't like Movim, I still invite you to find open platforms that can be tuned/hacked/adapted to your needs, there's plenty of them in the wild ;)